Particulate Matter Metrics, Adverse Health Outcomes and Ultrafine Particles
Historically, mass concentrations of airborne particulate matter have been the most widely used metric for assessing exposure levels. Part of the reason for this grew out of studies in the mid 1900’s that showed correlations between mass concentrations of coal dust and the debilitating lung disease of pneumoconiosis (“black lung”) in underground coal miners. Because mass was one of the easiest properties to measure scientists initially used this metric to study and quantify the levels of dusts and aerosols that are present in the air we breathe and it is still used today. But particulate matter poses a particularly complex hazard where dose and toxicity are not always correlated well with mass concentrations but are associated with other physical parameters such as number, surface area and shape, as well as composition. Particulate matter exposure assessment therefore needs to reflect the roles of these parameters, as well as chemistry and particle size and shape, relative to the broad range of adverse health problems that particulate matter creates.
Mass concentrations may have correlated well with the incidence rates of pulmonary pneumoconiosis and there may be other respiratory illnesses that show similar correlations today, but mass concentrations alone do not appear to explain many additional adverse health problems. While respiratory disease and cancer have been shown to be related to particulates for decades, additional diseases of the circulatory system and neurological disorders are now also being correlated with exposures to particulates. In some of these studies negative health outcomes still persist at very low mass concentrations that are generally deemed safe and there is little, if any, evidence of correlations with these low mass concentrations. For both respiratory and circulatory illnesses, the most convincing correlations appear to be with particulate matter surface areas, and, in many instances, a parameter called lung deposited surface area, or LDSA.
In addition, other recent studies that link particulate matter exposure to neurological disorders, such as dementia, show that number concentrations of nanometer size particles provide the best correlation. It is also worth noting that LDSA is also dominated by the presence of particles typically in the 20 nm to 400 nm particle size range and that high number and surface concentrations can occur at quite low mass concentrations. It is not unreasonable to expect such correlations since it is these very small particles that penetrate most deeply into the lung where they are retained with high efficiency. As more studies and more data accumulate that address the relationships between particulate matter exposure and adverse health outcomes, the scientific community needs to remain vigilant and cognizant of the importance of metrics other than mass that may be necessary to unravel these complex dependencies and interactions.
Measurement of ultrafine particulate matter, either as a component of PM2.5 or as particulate matter that exists as the only component of PM2.5, is more difficult because of the small size of the particles. Techniques that have been shown to work the best for measurement of these particles usually involve some sort of particle charging scheme where the particles flow through a region of molecular ions or where particles are charged directly, such as via irradiation by ultraviolet light or some combination of the different charging schemes. But these techniques are generally more expensive or involve radionuclides that render the technique difficult to implement.
Recently, Airviz Inc. has embarked upon a pioneering research effort to use simple and well-known optical techniques to measure ultrafine particle surface and mass concentrations that can result in small and inexpensive devices that will usher in a new era of particulate measurement capability currently unavailable to the general populace. In the envisaged end-unit device, which is expected to be smaller and more compact than the current Speck sensor, monitoring of mass and surface concentrations of ultrafine particles that exist by themselves or as a major/minor component of PM2.5 will become routine. This capability will allow us to expand our knowledge of the role(s) that ultrafine particles play in producing negative health consequences and premature morbidity. Just recently, for instance, a new study (see Ref. 5 below) indicated that for every increment in PM2.5 of 10 µg/m3 life expectancies can be reduced by as much as ten years. This is a remarkable number but questions still remain as to how much of this increase could be ultrafine particles and/or do ultrafine particles dominate these statistics. With improved measurement capabilities available to a wider spectrum of the population and a potentially much finer mesh for correlation of exposures, these questions may be finally answered. Stay tuned……
- https://link.springer.com/article/10.1007/s11051-005-6770-9
- http://dx.doi.org/10.1016/j.atmosenv.2016.04.019
- http://www.mdpi.com/1422-0067/17/11/1833
- http://www.sciencemag.org/news/2017/01/brain-pollution-evidence-builds-dirty-air-causes-alzheimer-s-dementia
- http://www.sciencedirect.com/science/article/pii/S1470160X17301693
Article by Dave Litton, Senior Scientist at Airviz Inc.
